Ion-Selective Electrodes With Ionophore-Doped Sensing Membranes

Ionophore-doped sensing membranes 2547
Cl
F 3 C
CF 3
Cl
CF 3
Cl
B
Cl
F 3 C
F 3 C
B
CF 3
N +
CF 3
CF 3
Figure 7
Common highly hydrophobic ionic sites used to dope ISE membranes.
Membrane
Sample
R − R − R − R − R − R −
K + K + K + K +
K + K +
K + Cl − K + Cl −
(a)
(b)
Figure 8 Equilibrium ion distribution between an aqueous KCl solution and an ISE membrane with a ratio of K + ionophore and
cationic sites of (a) 1 : 2, and (b) 2 : 1.
used in an optimized ratio to the ionophore can improve the
selectivity (Section 5.2). 53
Note that the concentration of ionic sites has to be low
enough so that the organic phase contains a substantial
concentration of free ionophore. For example, consider an
organic phase prepared to contain an anionic site and an
electrically neutral ionophore that binds K + with 1 : 1 stoichiometry.
If the molar ratio of ionic sites and ionophore
is 2 : 1 and the organic phase is equilibrated with an aqueous
KCl solution, all of the ionophore will be present in
the form of K + complexes, and only half of the K + ions
will be able to bind to an ionophore (Figure 8a). Because
of the very substantial concentration of uncomplexed K +
in the organic phase, the phase boundary potential will be
identical to the one observed for an ionophore-free organic
phase containing only ionic sites. On the other hand, if the
molar ratio of ionic sites and ionophore is 1 : 2 and the
organic phase is equilibrated with an aqueous KCl solution,
half of the ionophore will be present in the form of
K + complexes, half of the ionophore is in its uncomplexed
form (Figure 8b), and the phase boundary potential exhibits
the selectivity characteristic for an ionophore-doped organic
phase. This latter situation is similar to the one of pH
buffers, which only exhibit good buffering capacities if they
contain substantial concentrations of both an unprotonated
base and the conjugated acid (i.e., the proton complex of
the conjugated base). Following this analogy, we can simplify
the above statement that “a Nernstian ISE response
requires that the activity of the ion of interest in the bulk
of the water-immiscible sensing phase is constant and does
not depend on the sample.” More succinctly, a Nernstian
ISE response requires that the ionophore and the ionic sites
buffer the ion of interest in the sensing phase.
3.1.3 The conventional ISE measurement
As shown above, the phase boundary potential at the
interface between an aqueous sample and the hydrophobic
sensing phase of an ISE membrane depends logarithmically
on the activity of the ion of interest in the aqueous
sample (3). However, an experimental method to measure
this phase boundary potential directly does not exist. The
reason why potentiometric measurements, nevertheless,
exhibit the same logarithmic dependence on the activity of
the ion of interest in the sample is illustrated in Figure 9.
The actual potentiometric measurement determines the
EMF as the difference in the electrical potentials between
the connecting wire of the ISE and the connecting wire
of a reference electrode. As in every electrochemical
cell, the EMF is the sum of two types of components:
One type of contributions to the measured EMF arises
from the phase boundary potentials at all interfaces of
the electrochemical cell. Figure 9 illustrates the various
phase boundary potentials present along the path from the
copper connector of a typical ISE through the selective
electrode, the sample, and the reference electrode. These
phase boundary potentials include interfaces of different
types, such as metal–metal, metal–salt, salt–liquid, and
liquid–liquid interfaces. The other types of components
that contribute to the measured EMF of an electrochemical